1. Field of the Invention
The present invention relates to a liquid crystal device comprising ferroelectric liquid crystals as well as a backlight.
2. Description of the Related Art
Liquid crystal display devices utilizing the electro-optic effect of liquid crystals to display pixels have been developed. One such device, ferroelectric liquid crystal (FLC), is expected to be used in many applications because of its quick responsiveness. One of these applications, a domain graduation method, has been proposed by Philips in U.S. Pat. No. 4,840,462. In this method, in order to provide halftones in a TFT(thin film transistor)/FLC combination, the area of the domain inversion is varied by controlling the amount of electricity applied to pixel electrodes.
However, this method has problems because the bistable FLCs require black reset, That is, in TV driving, complicated signal processing is required, and in a small pixel, graduation levels available are limited by the size of the smallest white-inverted domains.
Another TFT/FLC method, employing FLC having a helix pitch, has been proposed in Japan Display '89 (1989), p.174, in which two states, i.e., the helical scatter state and the transparent state caused by voltage application, are alternated. This method has a problem in that different levels of voltage are required to coil and to uncoil the helix, causing hysteresis in the V-T (voltage-transmittance) characteristic.
A surface stabilized FLC device (referred to as "SSFLC device") proposed by Clark and Lagerwall in U.S. Pat. No. 4,367,924 is a bistable FLC device having two stable alignment states. With the bistable FLC cells, which invert from one state to the other when voltage is applied, halftone display is difficult, except in the domain graduation method as proposed by Philips.
Also, it is known that monostable mono-domain FLC cells are produced by aligning the molecules asymmetrically on upper and lower substrates, and that when DC voltage is applied to the cells so that dipoles of spontaneous polarization invert, the molecule axes rotate a certain amount. In this phenomena, domain inversion does not occur during the molecule axis rotation, so that complete halftones can be provided. However, an image will not be produced on an actual panel because the molecules turn back to the original state immediately when the applied writing pulses are discontinued. If appropriate DC field is maintained for a certain time, for example, by active switching units such as thin film transistors (TFT) provided with the cells, a desired image can be displayed. This technique is based on a principle that when a ferroelectric liquid crystal device having one stable state (hereinafter, referred to as "monostable FLC device") is supplied with a predetermined level of voltage, the FLC molecule axes turn according to the level of voltage, and when the applied voltage is discontinued, the molecules turn back to the original stable state by their own orientation force.
This technique will be described in detail.
With reference to FIG. 12, illustrating a sectional view of a liquid crystal cell of a liquid crystal device, an insulating film 1212 for preventing a short circuit between upper and lower electrodes is formed by the RF-sputter method on a glass substrate for liquid crystals 1210 having a transparent conductive film 1211. An orientation film 1213 is formed on the insulation layer 1212 by sintering polyimide which is applied to a thickness of approximately 5 nm by a spinner. After being sintered, the orientation film 1213 is rubbed by a normal method.
The other substrate 1222 has transistors composed of gate electrodes 1217, a gate insulating film 1218, source electrodes 1220, drain electrodes 1221 and a-Si semiconductor layers 1219, and display electrodes 1216 connected to the transistors. An insulating film 1215 for protecting the channels of the transistor portions is provided on top of this layered structure. On top of the insulating film 1215, a liquid crystal orientation film 1214 is formed in the same way orientation film 1213 is formed. The orientation film 1214 is also rubbed by a normal method after being sintered. The cell gap is maintained by a spacer 1223 having a grain size of about 1.7 .mu.m.
FIG. 13 shows one example of the driving waveforms in the device constructed as the above description. In this example, an information signal V.sub.OP is charged through a TFT to the cell when the gate pulse V.sub.G is on. Though the voltage V.sub.OP decays because of resistance in the liquid crystal layers, the liquid crystal molecule axes turn to let light pass, according to the voltage V.sub.OP. When the voltage V.sub.OP becomes zero, the molecule axes turn back to block light.
FIG. 14 illustrates the relation between the FLCs and a polarizer in the stable state without voltage being applied. When the polarization axes of the polarizer P and an analyzer A are perpendicular to each other and the directions of the polarizer P and the liquid crystal molecules in the stable state are the same, zero or negative voltage provides the black state. Positive voltage moves the molecules smoothly to a position, as shown by a broken line in FIG. 14, according to amount of positive voltage, and thus the light double-refracted by the polarization passes through the FLC device.
In an intermediate transmittance range, no domain inversion occurs so that complete halftones can be obtained. When the applied voltage is shifted to zero, the molecules return to the black state, i.e., the stable state, in less than several milliseconds since the liquid crystal cell is monostable. An absolute transmittance T in a double refraction cell is obtained from the following equation. EQU T=sin.sup.2 2.theta.sin.sup.2 (.DELTA.nd.pi./.lambda.)
where: .theta. is tilt angle (see FIG. 4); .DELTA.n is anisotropy of refractive index; d is cell thickness; and .lambda. is wave length. Since .DELTA.nd/.lambda. is approximately 1/2, the equation is approximately EQU T=sin.sup.2 2.theta.
The tilt angle providing the maximum transmittance in this example was 36.degree., so that the absolute transmittance in the liquid crystal portion is about 90% based on the equation. EQU T'=sin.sup.2 (2.times.36.degree.)=0.905
In the black state, the tilt angle never becomes greater in the negative direction even if a negative voltage is applied.
It is known that when the driving is carried out by the waveform shown in FIG. 13 and only positive voltage V.sub.LC is applied to the liquid crystal cell portions, the average luminance deteriorates as time passes (which is longer than the gate pulse interval). This luminance deterioration is different from the deterioration of the information signal voltage V.sub.OP caused by resistance of the liquid crystal layer and the like, shown in FIG. 13.
In order to reduce the luminance deterioration in the conventional art, a method can be applied, in which display a signal and non-display signal (the ground potential or a potential having a polarity different from that of the display signal) are applied alternatively in a constant cycle and constant periods by a voltage applying means. The display signal is the voltage according to image signals applied to the pixels. The non-display signal is signal voltage having no relation with the image.
However, when a black/white alternate driving method employing the non-display signal unit is used in a TV or the like, one pixel displays black half of the time. Therefore, there is a disadvantage in that the panel transmittance per period of one panel is half of that by means of a driving method without the non-display signal unit (the conventional art).
The bistable ferroelectric liquid crystal (FLC) device has two stable states in directions shifted a certain angle both ways from the axis direction (rubbing direction or the like) of the orientation operating surface, which is formed by the rubbing method or the like on the liquid-crystal-side surfaces of boards on both sides of the liquid crystal layer. The tilt angle is called a cone angle (hereinafter referred to by .theta.c). When voltage is applied perpendicularly to the liquid crystal layer of the FLC device, the FLCs shift from one stable state to the other. This shift corresponds to the 2.theta.c rotation of the optical axis of a material having anisotropy of refraction index. Therefore, when polarized light comes into the bistable FLC device having a thickness corresponding to the operation of a half-wavelength board, the circular polarization effect to the incoming polarized light in one stable state is 4.theta.c different from that in the other stable state. When the bistable FLC device is placed between polarizers (such as polarizing plates) of crossed nicols or parallel nicols, an ON/OFF ratio of transmitted-light quantity, i.e., transmittance ratio, contrast, between the two stable states reaches a maximum if 4.theta.c=90.degree.(.theta.c=22.5.degree.).
However, the cone angle is greatly dependent on the characteristics of a liquid crystal material and the orientating operation surface, so that an FLC device having a sufficient cone angle has yet to be developed. Thus, the modulation degree of the conventional art is insufficient for an optical modulating device.
One method to solve the above problem is described in U.S. patent application Ser. No. 673,070 (Mar. 21, 1991), in which two FLC devices capable of optical modulation and one half-wavelength plate are combined.
With this method, the cone angle of 11.25.degree. gives rise to the maximum ON/OFF ratio of transmitted light-quantity, i.e., the transmittance ratio, the contrast.
However, since each of the two liquid crystal devices is sandwiched between two transparent plates (which are usually glass plates having a thickness of about 1 mm), the distance between the two liquid crystal layers is greater than the size of the pixels in a high density display device; for example, when a liquid crystal device having a diagonal size of three inches is used in a projecting display apparatus for an EDTV, one side of a pixel in the device will be about 60 .mu.m. Therefore, problems occurs, such as decrease in the practical numerical aperture and light leakage from one pixel to another. The half-wavelength plate employed in the conventional art may also give rise to the same problems.